The performance of an electronic device can be affected by the environment of the device. RF components and circuit boards for RF components face a particular problem. The enclosure for an RF circuit needs to prevent any RF energy radiated by the circuit from escaping out of the enclosure and interfering with other circuitry of the device or with other nearby devices. Additionally, in most areas the FCC has specific limitations on how much RF energy the electronic device can release into the atmosphere. These limitations are meant to prevent an RF device from interfering with other devices.
The problem of preventing interference is even more significant for circuit boards that contain both RF and digital components. The RF energy radiated from the RF component is at a much higher frequency than the frequency used by the digital component. If this high frequency reaches the digital component it interferes with the much lower frequency of the clocks of the digital component, thereby disrupting the operation of the digital component. One problem caused by RF leakage is that the RF signals create false clock signals or edges for the digital component.
In an effort to minimize the interference, the enclosures for RF components are designed to shield and to attenuate as much of the RF energy as possible. The enclosures are fabricated from a material such as aluminum that balances the desire to keep the enclosure as inexpensive and lightweight as possible and the requirement to attenuate RF radiation. The enclosures are typically cast out of the aluminum into the desired shape.
FIG. 1 shows an assembly that has both RF component 12 and digital component 14 on circuit board 16 attached to a conventional enclosure and to back plate 17. FIG. 2 shows a top view of enclosure 20 of FIG. 1. Referring to FIGS. 1 and 2 concurrently, enclosure 20 has plate 22, wall 24 that surrounds all of the components, and wall 26 that separates RF component 12 and digital component 14. Wall 24 has openings 15.sub.1, 15.sub.2, 15.sub.3 through which power, ground, and any other connections are supplied to components 12, 14 and circuit board 16. Wall 24 has lower portion 34 and upper portion 27. Circuit board 16 fits against lower portion 34. Enclosure 20 has standoffs 28.sub.1 . . . 28.sub.15, for fasteners, typically screws, that attach enclosure 20 to circuit board 16. The tops of standoffs 28.sub.1 . . . 28.sub.15 fit through openings in circuit board 16. Optionally, enclosure 20 also includes tuning openings 30.sub.1 . . . 30.sub.6 for inserting testing and tuning instruments to test and tune the components after the assembly is encased in enclosure 20. Once the testing and tuning is complete the tuning openings 30.sub.1 . . . 30.sub.6 are covered with electromagnetic interference (EMI) tape, which is a thin aluminum tape used to prevent any RF energy from escaping through the tuning openings 30.sub.1 . . . 30.sub.6.
Enclosure 20 should avoid any gaps between walls 24, 26 and circuit board 16 because RF energy escapes through these gaps. To prevent gaps there is a large amount of torque on the screws that hold circuit board 16 and enclosure 20 together. Additionally, the screws are placed close to walls 24, 26 to tightly fasten circuit board 16 to enclosure 20. The torque on the screws is typically large enough that if the screws were not placed in standoff 28.sub.1 . . . 28.sub.15 that absorb some of the force of the screws, the screws could bend plate 22 of enclosure 20.
A problem with conventional enclosures is that even if circuit board 16 is tightly pressed against enclosure 20 too much RF energy passes around each of walls 24 and 26.
Additionally, the tolerance of the metal may still allow for gaps between walls 24, 26 and circuit board 16. As shown in FIG. 3, in an effort to eliminate these gaps, RF gasket 32 can be placed on wall 26. RF gasket 32 is a silver filled silicone rubber gasket. Referring to FIGS. 1 and 4, RF gasket 32 has groove 36. Groove 36 allows two sides 37, 38 of RF gasket 32 to fit around, a wall or around a notch in the wall, so that RF gasket 32 surrounds an end of the wall. This prevents RF energy from escaping around any gaps between the end of the wall and circuit board 16. Unfortunately, since lower portion 34 of wall 24 is attached to upper portion 27 and there are no notches in lower portion 34, RF gasket 32 cannot be easily placed against lower portion 34 to assist in eliminating any gaps between wall 24 and circuit board 16.
Referring to FIGS. 3 and 4, another problem with conventional enclosure 20 is that with conventional casting equipment if two shapes are not located far enough apart the area between them is filled solid. Standoffs 28.sub.1 . . . 28.sub.13 should be in locations where the screws through standoffs 28.sub.28 . . . 28.sub.13, fasten enclosure 20 tightly enough to circuit board 16 to try to prevent gaps. This typically locates standoffs 28.sub.1 . . . 28.sub.13 close enough to walls 24, 26 that the areas between standoffs 28.sub.1 . . . 28.sub.13 and walls 24, 26 are filled with metal connections 36.sub.1 . . . 36.sub.13. The metal connections 36.sub.1 . . . 36.sub.13 do not allow RF gasket 32 to be placed on the portion of wall 26 attached to the metal connections.